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A graphic that shows the difference of a normal groundwater table and a lowered one. Courtesy of Boston Groundwater Trust.

Holding up many of the buildings that are found in neighborhoods like the Back Bay and flat of Beacon Hill are underground wood pilings that provide the support needed to keep them from sinking into the marshlands that once surrounded Boston.

But, with a drought in effect, the water table that keeps the wood pilings submerged underwater is slowly receding and this may expose the wood to oxygen. Once, open to the air, microbes come in that eat at the wood, causing it rot.

“The good news is – it takes time,” said Christian Simonelli the executive director of the Boston Groundwater Trust for the rot to set in. “The other good news is once we get rain fall and we will eventually – the process will stop.”

The only source of ground water comes from rainfall.

There are about 800 wells that serve as the warning system to the water table that Boston Groundwater Trust monitors about every five to six weeks.

In the past three to four months they have been seeing the level slowly go down about a few inches each month. Typically, during this time of year Simonelli said they typically have 26 to 27 inches in the well but right now they have only around 18 inches. On average it is about five to six inches lower than normal.

Due to the differing soil content between each neighborhood, some may see more of a drop than others.

“Essentially this has been a prolonged period without rainfall,” said Simonelli. “The whole year has been below average. We had not had what we typically have.”

The original peninsula that the Puritans resided on was so small that from an early date they began to make new build-able land. This land, that can be found around most neighborhoods in Boston today was formed by dumping sand and gravel on top of the mud flats that where once there.

This new land is not strong enough to support heavy structures including the multistory brick row houses that line many of the historic neighborhoods in Boston.

Therefore, pilings where stuck into the man made land down 30 to 40 feet below the ground surface to support structures that reside on top of them. Nearly all of the buildings made in the early part of the 20th century are supported by wood pilings.

These pilings will last centuries if they remain submerged in groundwater, but if the groundwater drops, the tops of the piles are attacked by microbes and eventually rot, which can lead to severe foundation problems for the building.

Older buildings that sit on top of these wood pilings can be found in Fenway, Back Bay, South End, Bay Village, the flat of Beacon Hill, Chinatown, the North End, downtown waterfronts and the Fort Point Channel section of South Boston, along with sections of East Boston.

If the pilings fail the buildings will start to settle and a few cracks will appear. Over time the windows and doors will become unusable and the floors will no longer be level. In worst cases the building will no longer be safe to occupy.

In the years since 1929, close to 200 buildings in Boston have had to have their pilings repaired. The cost to repair a typical three or four story row house can be $400,000 with larger buildings costing a lot more.

There are a few areas that Boston Ground Water deems as hot spots in different neighborhoods around Boston, but Simonelli said they are mostly due to infrastructure problems.

If precipitation has been low, they typically see the water table fall a few inches, if it is falling by feet they know that there is something wrong structurally such as a leak in a pipe that needs to be fixed.

For now, there are no quick solutions other than to wait for the rain.

“Hopefully it rains at night and not on the weekend,” joked Simonelli. “But we definitely need the rain, that’s for sure.”